abcd_Server1 CCDDomain
defg_Server1 GGFDomain
kdkhs_Server1 CCDDomain
abce_Server1 CCDDomain
dgdg_Server1 CCADomain
dfdkhs_Server1 GGFDomain

grep -oh '*Domain' "ServerNames.txt" | sort -u | wc -l

$ awk '$2 ~ /Domain/ && !a[$2]++{ cnt++ }END{ print cnt }' ServerNames.txt
3

$ uniq -f1 <(sort -k2 ServerNames.txt) | wc -l
3

grep -oh '[[:alpha:]]*Domain[[:alpha:]]*' ServerNames.txt | sort | uniq | wc -l

% grep -oh "wDomainw" ServerNames.txt
CCDDomain
GGFDomain
CCDDomain
CCDDomain
CCADomain
GGFDomain

% grep -oh 'wDomainw' ServerNames.txt | sort -u | wc -l
3